The following calculations and considerations are intended to serve as a contribution to the attempt to detect nuclear fuel reprocessing activities through 'environmental' measurements. As a result of the reprocessing process, the fission products xenon and krypton are released to the atmosphere. They show a different isotopic composition compared to natural atmospheric xenon (krypton). The released fission xenon (krypton) is diluted in air, mixed with natural xenon (krypton) and therefore results in a change of isotopic composition. Consequently, the question arises whether it is possible to detect the small isotopic alterations in atmospheric xenon (krypton) and the detection limit thereof. The calcualtions performed in this context are based on the principles of isotope mixing as in dilution. In order to provide an estimation of the isotopic alterations in atmospheric xenon (krypton) blendingcalculations were carried out using the isotopic compositions of atmospheric xenon (krypton) and calculated compositions of fission gases. Subsequently, the calculated ratios in the blend were compared to the ratios of natural xenon as measured using the highly developed, ultra-accurate measurement facilities on isotopic mass spectrometry at the IRMM.